1. Field of the Invention
This invention relates to video signal processing, and more particularly, but not exclusively, to methods of and apparatus for digital video sample mapping. This technique is used, for example, in digital video effects units, in which digital television signals are manipulated to give special effects.
2. Description of the Prior Art
Such special effects are now well known to television viewers, and enable images on the cathode ray tube of a television receiver to be, for example, off-set (moved in any direction), scaled (expanded or compressed in size), rolled (rotated) in two or three dimensions, and so on. Such special effects can be generated by mapping video sample values from one sample domain to another with a dynamically changing mapping function.
UK patent specification No. GB-A-1 594 341 discloses a processing system capable of effecting relatively low-speed scaling of an image using a technique similar to that used in a television standards converter.
However, consider the example shown in FIG. 1 of the accompanying drawings. In an input array A, sample values a.sub.n-2 to a.sub.n+3 correspond to six successive sample positions along a horizontal scan line of an input digital television signal. Suppose that the requirement is to compress at least that part of the picture in the ratio 4:3. Mere compression would simply move the sample values a.sub.n-2 to a.sub.n+3 to more closely spaced points in an array B'. However, the required output array B in fact consists of sample positions identical with those of the input array A, which is not the case in television standards conversion, so it is further necessary to map the sample values of the array B' onto the output array B, so that the sample values have the correct magnitudes and are in the correct positions.
A relatively simple solution to this problem is to use for each sample position in the output array B that sample value from the array B' which is spatially nearest. Obviously this results in inaccuracies, and other more sophisticated solutions have been proposed, although these tend to involve complex calculations and to require a large amount of storage, particularly where more than one dimension is involved. Also, the previously proposed solutions tend not to operate satisfactorily where the mapping function is changing rapidly.